1. Field of the Invention
The present invention relates to organic transistors, organic transistor arrays, and display apparatuses.
2. Description of the Related Art
In the recent years, organic thin-film transistors that use organic semiconductor materials are being vigorously studied. Advantages of such organic thin-film transistors include diversity in material composition; flexibility in manufacturing process and product form; the ease with which areal increases can be achieved; simplicity of manufacturing process based on a simple layered structure; and inexpensive manufacturing equipment.
Organic thin-film transistors can be easily manufactured in the form of thin-films or circuits using a print method, a spin-coat method, a dipping method, etc, at a cost far smaller than the cost of manufacturing thin-film transistors using conventional Si semiconductor material. Integration of organic thin-film transistors requires the patterning of an organic semiconductor layer. If transistors are integrated without patterning of an organic semiconductor layer, off-current increases during the operation of the transistors, resulting in an increase in power consumption. The increase in off-current may also cause crosstalk when the transistors are used to drive a display medium. When patterning a semiconductor layer using Si semiconductor material, photolithography and etching are employed. Specifically, after a photoresist is applied and a desired pattern is exposed and developed to form a resist pattern, etching is performed using the resist pattern as an etching mask. The resist is then removed, thereby producing an etched pattern.
Photolithography and etching may also be employed for the patterning of an organic semiconductor layer, in the same way as in the case of Si semiconductor material. However, when a polymer material is used as an organic semiconductor material, if a photoresist is applied to a polymer organic semiconductor layer to form a pattern or the like, transistor characteristics may deteriorate. This is due to the fact that polymer materials are in many cases dissolved by an organic solvent contained in a photoresist, which generally consists of a novolac resin having naphthoquinone diazide as a photosensitive group that is dissolved in an organic solvent (such as xylene or cellosolve solvent). When a low-molecular crystalline material, such as pentacene, is used as an organic semiconductor material, deterioration in transistor characteristics may be observed to varying degrees. Further, such a material may be damaged by the resist stripper (such as ethyleneglycol monobutyl ether, or mono ethanol amine), which is used for removing the resist. The material may also be damaged by rinsing with pure water after the resist is removed.
In a method of patterning an organic semiconductor layer using a crystalline material, such as pentacene, a shadow mask is used. However, this method is limited by pattern size, and is not suitable for patterning of large areas. In addition, a shadow mask has a certain operating life. Thus, it is very difficult with this method to achieve a drastic cost reduction compared with the manufacture of thin-film transistors using Si semiconductor material.
Inkjet printing is a viable method of patterning an organic semiconductor layer. Japanese Laid-Open Patent Application No. 2004-297011 discloses a method of manufacturing an organic transistor based on an appropriate combination of a method whereby an electric charge is imparted at a predetermined position on a surface to be coated, and a charge having an opposite polarity is imparted to a coating material so that the charged material can be guided by Coulomb force to the predetermined position; a method whereby a concave portion is formed at a predetermined position on a surface to be coated in order to deposit a coating material in the concave portion; and a method whereby, after application of a material, solvent is evaporated to form a pattern, followed by irradiating the pattern with laser. Japanese Laid-Open Patent Application No. 2004-141856 discloses a patterning method whereby an indented region is formed on the surface of a substrate, and a liquid material is deposited on the surface at a selected location adjacent the indented region to form a pattern.
Because such an inkjet print method makes it possible to directly draw a pattern, dramatic improvements in material utilization can be achieved. Thus, an inkjet print method of patterning an organic semiconductor layer may potentially simplify manufacturing process, increase throughput, and reduce cost.
When a polymer material that is soluble in an organic solvent is used as an organic semiconductor material, an organic semiconductor ink can be prepared by dissolving such a polymer material in an organic solvent, so that an inkjet print method can be used for patterning. However, in this method, it is difficult to form a pattern with resolution of 50 μm or smaller, considering the printing accuracy, so that it is difficult to achieve higher resolution than possible with photolithography. One possible solution may be to reduce the size of ink droplets; this, however, is technically difficult and associated with stability problems, including the increased likelihood of blocked discharge or skewed discharge.
Furthermore, when an area to be patterned is large, it is very difficult to pattern every transistor satisfactorily, due to landing accuracy, for example. In particular, because the physical properties of an organic semiconductor ink (such as viscosity, surface tension, and drying property) vary depending on the physical properties (such as purity, molecular weight, and molecular weight distribution) of the polymer material used or the type of organic solvent, it is difficult to achieve an appropriate physical property of such an ink. As a result, not all of the nozzles used in the inkjet method may be able to discharge properly; some nozzles may discharge in a skewed manner or with different volumes. The same applies to the head characteristics; namely, the nozzles do not necessarily have identical head characteristics. Should one of the nozzles develop even a small amount of discharge skew, patterning at higher resolutions may fail even when patterning can be successfully performed at lower resolutions. As a result, when an island-shaped organic semiconductor layer is desired to be formed, an incomplete pattern (see FIG. 9B) may result instead of a fine-resolution pattern (see FIG. 9A). In the organic transistor array shown in FIGS. 9A and 9B, there are successively formed on a substrate (not shown) a gate electrode 2; a gate insulating film 4; a source electrode 5 and a drain electrode 6; and an organic semiconductor layer 7.